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[ty] Fix bug in union builder optimization #22085

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[ty] Fix bug in union builder optimization #22085

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07d3527
[ty] Skip redundancy checks for sub-union-elements
MichaReiser Dec 19, 2025
7b6d0a9
Apply same check for literals
MichaReiser Dec 19, 2025
6a2f787
[ty] Fix sub-union optimization to require both flags be No
claude Dec 19, 2025
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83 changes: 67 additions & 16 deletions crates/ty_python_semantic/src/types/builder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -300,7 +300,7 @@ impl<'db> UnionBuilder<'db> {
}
}
for ty in replace_with {
self.add_in_place_impl(ty, seen_aliases);
self.add_in_place_impl(ty, seen_aliases, None);
}
}

Expand All @@ -312,10 +312,15 @@ impl<'db> UnionBuilder<'db> {

/// Adds a type to this union.
pub(crate) fn add_in_place(&mut self, ty: Type<'db>) {
self.add_in_place_impl(ty, &mut vec![]);
self.add_in_place_impl(ty, &mut vec![], None);
}

pub(crate) fn add_in_place_impl(&mut self, ty: Type<'db>, seen_aliases: &mut Vec<Type<'db>>) {
pub(crate) fn add_in_place_impl(
&mut self,
ty: Type<'db>,
seen_aliases: &mut Vec<Type<'db>>,
skip_checks_after: Option<usize>,
) {
let cycle_recovery = self.cycle_recovery;
let should_widen = |literals, recursively_defined: RecursivelyDefined| {
if recursively_defined.is_yes() && cycle_recovery {
Expand All @@ -329,8 +334,22 @@ impl<'db> UnionBuilder<'db> {
Type::Union(union) => {
let new_elements = union.elements(self.db);
self.elements.reserve(new_elements.len());
// Capture the current element count to avoid comparing union elements against each other.
// The union has already been simplified, so its elements don't need redundancy checks
// between themselves, only against pre-existing elements. However, we only apply this
// optimization when not in cycle recovery AND both the builder and union have
// recursively_defined=No (indicating neither is involved in recursive type definitions
// where simplification might be incomplete).
let batch_start = if !self.cycle_recovery
&& self.recursively_defined == RecursivelyDefined::No
&& union.recursively_defined(self.db) == RecursivelyDefined::No
{
Some(self.elements.len())
} else {
None
};
for element in new_elements {
self.add_in_place_impl(*element, seen_aliases);
self.add_in_place_impl(*element, seen_aliases, batch_start);
}
self.recursively_defined = self
.recursively_defined
Expand All @@ -355,7 +374,9 @@ impl<'db> UnionBuilder<'db> {
// leave out the recursive alias. TODO surface this error.
} else {
seen_aliases.push(ty);
self.add_in_place_impl(alias.value_type(self.db), seen_aliases);
// Don't pass through skip_checks_after when unpacking aliases, since the alias
// value is not part of the original union batch.
self.add_in_place_impl(alias.value_type(self.db), seen_aliases, None);
}
}
// If adding a string literal, look for an existing `UnionElement::StringLiterals` to
Expand All @@ -371,13 +392,16 @@ impl<'db> UnionBuilder<'db> {
UnionElement::StringLiterals(literals) => {
if should_widen(literals.len(), self.recursively_defined) {
let replace_with = KnownClass::Str.to_instance(self.db);
self.add_in_place_impl(replace_with, seen_aliases);
self.add_in_place_impl(replace_with, seen_aliases, None);
return;
}
found = Some(literals);
continue;
}
UnionElement::Type(existing) => {
// Skip redundancy checks against elements from the same union batch.
if skip_checks_after.is_some_and(|batch_start| index >= batch_start) {
continue;
}
// e.g. `existing` could be `Literal[""] & Any`,
// and `ty` could be `Literal[""]`
if ty.is_subtype_of(self.db, *existing) {
Expand Down Expand Up @@ -420,13 +444,16 @@ impl<'db> UnionBuilder<'db> {
UnionElement::BytesLiterals(literals) => {
if should_widen(literals.len(), self.recursively_defined) {
let replace_with = KnownClass::Bytes.to_instance(self.db);
self.add_in_place_impl(replace_with, seen_aliases);
self.add_in_place_impl(replace_with, seen_aliases, None);
return;
}
found = Some(literals);
continue;
}
UnionElement::Type(existing) => {
// Skip redundancy checks against elements from the same union batch.
if skip_checks_after.is_some_and(|batch_start| index >= batch_start) {
continue;
}
if ty.is_subtype_of(self.db, *existing) {
return;
}
Expand Down Expand Up @@ -468,13 +495,16 @@ impl<'db> UnionBuilder<'db> {
UnionElement::IntLiterals(literals) => {
if should_widen(literals.len(), self.recursively_defined) {
let replace_with = KnownClass::Int.to_instance(self.db);
self.add_in_place_impl(replace_with, seen_aliases);
self.add_in_place_impl(replace_with, seen_aliases, None);
return;
}
found = Some(literals);
continue;
}
UnionElement::Type(existing) => {
// Skip redundancy checks against elements from the same union batch.
if skip_checks_after.is_some_and(|batch_start| index >= batch_start) {
continue;
}
if ty.is_subtype_of(self.db, *existing) {
return;
}
Expand Down Expand Up @@ -524,30 +554,42 @@ impl<'db> UnionBuilder<'db> {
.all(|name| enum_members_in_union.contains(name));

if all_members_are_in_union {
// Don't pass through skip_checks_after when converting to enum class instance,
// since this is a transformation and not part of the original union batch.
self.add_in_place_impl(
enum_member_to_add.enum_class_instance(self.db),
seen_aliases,
None,
);
} else if !self
.elements
.iter()
.filter_map(UnionElement::to_type_element)
.any(|ty| Type::EnumLiteral(enum_member_to_add).is_subtype_of(self.db, ty))
{
self.push_type(Type::EnumLiteral(enum_member_to_add), seen_aliases);
self.push_type(
Type::EnumLiteral(enum_member_to_add),
seen_aliases,
skip_checks_after,
);
}
}
// Adding `object` to a union results in `object`.
ty if ty.is_object() => {
self.collapse_to_object();
}
_ => {
self.push_type(ty, seen_aliases);
self.push_type(ty, seen_aliases, skip_checks_after);
}
}
}

fn push_type(&mut self, ty: Type<'db>, seen_aliases: &mut Vec<Type<'db>>) {
fn push_type(
&mut self,
ty: Type<'db>,
seen_aliases: &mut Vec<Type<'db>>,
skip_checks_after: Option<usize>,
) {
let bool_pair = if let Type::BooleanLiteral(b) = ty {
Some(Type::BooleanLiteral(!b))
} else {
Expand Down Expand Up @@ -585,10 +627,16 @@ impl<'db> UnionBuilder<'db> {
}

if Some(element_type) == bool_pair {
self.add_in_place_impl(KnownClass::Bool.to_instance(self.db), seen_aliases);
self.add_in_place_impl(KnownClass::Bool.to_instance(self.db), seen_aliases, None);
return;
}

// Skip redundancy checks when comparing elements from the same union batch.
// The union has already been simplified, so its elements don't need to be checked
// against each other.
let skip_redundancy_checks =
skip_checks_after.is_some_and(|batch_start| i >= batch_start);

// Comparing `TypedDict`s for redundancy requires iterating over their fields, which is
// problematic if some of those fields point to recursive `Union`s. To avoid cycles,
// compare `TypedDict`s by name/identity instead of using the `has_relation_to`
Expand All @@ -597,7 +645,10 @@ impl<'db> UnionBuilder<'db> {
continue;
}

if should_simplify_full && !matches!(element_type, Type::TypeAlias(_)) {
if should_simplify_full
&& !skip_redundancy_checks
&& !matches!(element_type, Type::TypeAlias(_))
{
if ty.is_redundant_with(self.db, element_type) {
return;
}
Expand Down
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